Cathode-ray tube for flying-spot scanning

ABSTRACT

A cathode-ray tube for flying-spot scanning devices in which the cathode-ray screen contains as a phosphor a yttrium aluminate activated with trivalent cerium.

United States Patent [72] inventors Georgelilasse;

Alfred Bril, Emmasingel, Eindhoven,

' Netherlands [21] App]. No. 723,551 [22] Filed Apr. 23, 1968 [45] Patented Feb. 16, 1971 [73] Assignee U.S. Philips Corporation New York, N.Y. [32] Priority Apr. 29, 1967 [33] Netherlands [31] 6,706,095

[54] CATHODE-RAY TUBE FLYING-SPOT [51] lnt.Cl H01j1/63,

H. R. Lewis Paramagnetic Resonance of Ce 4" IInunij-arn! lournnluj lppllm I'huxlu lol. 31 .\'o. 2 Feb. 1966 panes 139 41 Copy Pat. Office Search Center Primary Examiner-Tobias E. Levow Assistant Examiner-R. D. Edmonds Attorney-Frank R. Trifari ABSTRAC'BA cathode-ray tube for flying-spot scanning devices in which the cathode-ray screen contains as a phosphor a yttrium aluminate activated with'trivalent cerium.

'1 CATHODE-RAY TUBE FOR FLYING- SPOT SCANNING The invention relates to a cathode-ray tube, especially a cathode-ray tube for flying-spot scanning devices, for example, for flying-spot scanning of films or transparencies. The invention also relates to a phosphor for use in the screen of such a cathode-ray tube.

For displaying films or transparencies by means of television systems, the films or transparencies are usually scanned with a light beam produced by a rapidly moving light spot on the screen of a cathode-ray tube. The light emitted by the moving spot is protected onto the transparency or film and the transmitted light falls on'a photoelectric cell in which the energy of the transmitted part of the light is converted into a an electric signal. This electric signal is transmitted in the usual manner by a communication system, for example, a television transmitter.

Such a system has been used for the transmission not only of information from films o'r transparencies but also of information from transparent 'objects, wh'ich may be disposed under a microscope, or from opaque objects, for example securities, such as checks, or other printed or written documents. With opaque objects the rays reflected by them are projected'onto a photocell.

For a correct reproduction of the information from the object scanned, the decay time of the phosphor on the screen of the cathode-ray tube must be shorter than the time in which the electron beam generating the flying spot moves a distance equal to a full diameter on the screen, for at each instant the light produced by that part of the screen which was excited immediately before the respective'instant must be at least sub stantially extinguished. Otherwise the information displayed at this instant would be mixed with information from the parts scanned prior to this instant.

The term decay time of the phosphor" as used herein is to be understood to mean the time in which the intensity of the light on termination of the electron bombardment decreases to He of the maximum intensity'produced before.

When the electron beams moves at a high speed, for example, at the speed standardized for television transmission systems, the decay time computed according to the above rules must be less than approximately 10- second.

For the display of the information from noncolored objects, for example, films or transparencies, in black and white, several materials are known which satisfy the aforementioned requirements. One of the commonly used materials is calcium aluminum silicate activated with trivalent cerium (gehlenite). Another suitable material is magnesium calcium silicate activated with trivalent .ceriu rn. The former material has an emission substantially in the blue part of the spectrum with a maximum between 400 and 420 nm.; the latter material has an emission in the ultra-violet part of the spectrum with a maximum at approximately 380 nm. The decay time of both materials is less than l-" second.

These materials are unsuitable for use in cathode-ray tubes for flying-spot scanning devices for the transmission of the information from colored objects, for example, color films or color transparencies. For this purpose the screen of the cathode-ray tube must include a phosphor the emission of which extends preferably over the entire visible part of the spectrum or at least over a large portion thereof. This is related to the manner'in which the information from the colored objects is obtained and transmitted. This may .briefly be summed up as follows. I

The object is scanned with the light generated by the cathoderay tube. The radiation transmitted or reflected by the object is separated into a red, a green and a blue component by means of dichroic mirrors and/or filters. Each of these components falls upon a separate photoelectric cell. The electric signals produced by these photocells are combined and transmitted in the usual manner. Naturally in thisprocess it is necessary for the phosphor in the cathode-ray tube by which the flying-spot of illumination is produced to emit radiation in the entire visible part of the spectrum, for the transmission or reflection of a colored object also depends upon the wavelength of the radiation with which it is scanned.

The above applies not only to the transmission systems in which a colored image is composed at the display end, for example, a color television system, but also to the systems in which a colored object is displayed only in black and white. For the correct gradation in black and white, which must correspond to that of sthe colored objects, it is also of importance for the emission of the phosphor in the cathode-ray tube of the flying-spot scanner to extend over a large part of the spectrum.

The aforementioned known phosphors do not satisfy the requirements for producing the correct information from colored objects. Hence another phosphor, namely green-luminescing zinc oxide, was used, although properly speaking its decay time of l() second is too long. The spectral energy distribution of this material extends into the blue and red parts of the spectrum, although in the extreme blue and red parts the energy emitted is much less than in the central part.

In addition, as the requirements demanded of a true reproduction of colored objects, especially color films and color transparencies, are steadily raised, the zinc oxide is not satisfactory because the intensity of the luminescent light emitted in the red part of the spectrum is too low. At the red end of the spectrum this effect is enhanced, because in the commonly used photocells the sensitivity to red radiation is appreciably less than that to blue radiation.

A cathode-ray tube in accordance with the invention, especially for use in flying-spot scanning devices, has a target screen including a phosphor consisting of yttrium aluminate activated with trivalent cerium which satisfies the formula i. L -'i l i On excitation by electrons a material of the above formula which has garnet structure exhibits a broad emission spectrum which extends from about 470 nm. to about 720 nm. with a maximum at about 550 nm. Thus, at the long-wave end of the spectrum the emission of the garnet extends considerably further than the emission of the garnet extends considerably further than the emission of the zinc oxide used hitherto. This enables the red colors also to be correctly transmitted.

The decay time of a garnet satisfying the above formula is less than 10- second. In addition, the efficiency of the conversion of the energy of the incident electrons into light is high and in any case equal to that of the zinc oxide commonly used hitherto.

Preferably this yttrium aluminate satisfies the formula When x is greater than 0.20 concentration extinction occurs; when x is less than 0.01 the efficiency in the visible part of the emission spectrum falls off too much for practical use.

Generally attempts will be made to increase the emission of the luminescent screen of a cathode-ray tube in accordance with the invention in the blue part of the spectrum by including in the screen a further phosphor the emission of which lies in the blue part of the spectrum. Such a material may be admixed to the garnet, but it may also be provided on the screen as a separate layer. Obviously such a material also must have a short decay time of less than 10- second. This second material may be, for example, the above-described known gehlenite.

This combination permits a perfect reproduction of the information of the colored objects.

The manufacture of the garnet in accordance with the invention does not give rise to more than the usual difficulty. as ,will be described more fully hereinafter. lts incorporation in the screen of the cathode-ray tube also may take place in the usual manner.

The invention will now be described more fully with reference to an example of manufacture and a drawing.

In the drawing, H6. 1 shows schematically a cathode-ray tube in accordance with the invention; and

H6. 2 is a graph showing the spectral energy distribution of a phosphor produced according to the following example:

EXAMPLE 16.85 g. of Ygoa 20.00 g. of M 0 with an aluminum content of 33.7 percent by weight and 0.49 g. of Ce2O are mixed.

The mixture is heated in air to a temperature between l,250 C. and l,350 C. for 2 hours. The resulting reaction product is ground and homogenized and then heated in air to a temperature between l,400 C. and 1,600 C. for 2 hours. In the heating operations, the air may be replaced by oxygen or nitrogen or by an atmosphere of nitrogen containing 5 percent by volume of hydrogen. After the second heat treatment and, if required, after homogenizing and straining, the phosphor is ready for use. It may be provided in the screen of a cathoderay tube shown schematically in FIG. 1 in the usual manner, for example, with the aid of a binder.

In FIG. 1, reference numeral 1 denotes the wall of the conical portion of a cathode-ray tube in accordance with the invention. At its wide end, the conical portion is closed by a glass part 2 which serves as the support of a luminescent screen 3. The luminescent screen includes yttrium aluminate activated with trivalent cerium according to the invention.

FIG. 2 is a graph showing the wavelength in nanometers as abscissae and the intensity of the luminescent radiation as ordinates. The curve 4 shows the emission spectrum of the material produced according to the above example. The maximum of this emission is given the value 100. For comparison a curve 5 shows the emission spectrum of the zinc oxide used hitherto.

The material produced according to the above example, was found to give an efficiency of 3.6 percent in the visible part of the spectrum when excited with electrons of 20 k. The decay time was less than lO- second.

We claim:

1. A cathode-ray tube particularly suitable for use in flyingspot scanning devices, said tube comprising a discharge tube having a vacuum tight envelope, an electron gun provided within said envelope for producing an electron beam and a luminescent screen capable of being excited by said electron beam, said luminescent screen containing a trivalent cerium activated yttrium aluminate phosphor of the formula (Y,CC)3A|5O12;

2. The cathode-ray tube of claim 1 wherein the trivalent cerium activated yttrium aluminate is of the formula Y ,Ce, Ago wherein 0.01 x 0.2(

3. The cathode-ray tube of claim 1 wherein the luminescent screen includes, in addition to the trivalent cerium activated yttrium aluminate a phosphor having a decay time of less than 10- second and an emission in the blue part of the spectrum.

4. The cathode-ray tube of claim 3 wherein the phosphor having an emission in the blue part of the spectrum is gehlenite. 

2. The cathode-ray tube of claim 1 wherein the trivalent cerium activated yttrium aluminate is of the formula Y3-xCexA15012 wherein 0.01 < x < 0.20.
 3. The cathode-ray tube of claim 1 wherein the luminescent screen inCludes, in addition to the trivalent cerium activated yttrium aluminate a phosphor having a decay time of less than 10-7 second and an emission in the blue part of the spectrum.
 4. The cathode-ray tube of claim 3 wherein the phosphor having an emission in the blue part of the spectrum is gehlenite. 